Chromium plating



2,800,437 CHROMIUM PLATING Jesse E. Stareck, Royal flak, and Edgar J. Seyb, Jr., Oak Park, Mich, assignors, by mesne assignments, to Metal & Thermit Corporation, New York, N. Y., a corporation of New Jersey No Drawing. Application July 26, 1955, Serial No. 524,579

14 Claims. (Cl. 204-37) This invention relates to an improved method for chromium plating hard, high strength steels. Conventional chromium plating of such steels is accompanied by such a high loss of fatigue strength that failure of the steel usually occurs under repeated loading; moreover, the loss in fatigue strength increases with increasing strength of the steel. Baking of the plated steel, such as is customary to help counteract the effect of hydrogen embrittlement, further reduces the fatigue strength, the reduction being more marked as the thickness of the plate increases.

This unfavorable eifect of chromium plating has naturally limited the usefulness of these plated steels, and although attempts have been made to lessen the effect, for one reason or another no practical method has evolved for producing a satisfactory result. It is a principal object, of the invention to provide a method for chromium plating hard, high strength steel without substantial loss of fatigue strength comprising plating the steel in a mixed catalyst plating bath of the kind, and under the conditions, described hereinafter and then baking the plated steel at 200 to 800 F. The present bath produces a deposit on the steel which, on subsequent baking, enables the fatigue strength to be restored to values approaching that of the original unplated steel, with the amount of strength restoration increasing with the baking temperature. The resultant chromium plated, heat treated steel enjoys the protection against corrosion and wear, and the hardness and low resistance to friction, that the chromium deposit affords, and by virtue of the present improvement is suitable for use under service conditions of stress and strain that approach the fatigue limit, particularly where elevated temperatures are encountered. Thus the field of application of the plated steel is enlarged.

The method, in brief, comprises electrodepositing chromium on an article of steel having a fatigue strength of at least 50,000 p. s. i. (pounds per square inch), particularly 60,000 or 70,000 to 110,000 p. s. i., although steels of higher strength may also be plated. A current of 1 to 6 a. s. i. (amperes per sq. inch) is passed to the article as cathode in an aqueous chromium plating bath at a temperature of 120 to 170 F. The bath comprises essentially 400 to 800 g./l. of CrO3, 0.5 to 6.0 g./l. of dissolved sulfate, 804 and from a tenth or more, say 0.1 to 0.5, up to 16.0 g./l. of dissolved silicofluoride, SiFtF. The catalyst radicals are supplied by suitable sulfate-containing and silicofluoride-containing compounds as hereinafter described. The sum of dissolved sulfate and dissolved silicofluoride varies with the CrOs concentration as follows: as the CrOs increases from a lower limit of 400 g./l. to an upper limit of 800 g./l., the lower limit of said sum increases linearly from 2.7 to 4.4 g./l. while the upper limit of the sum increases linearly from 9.0 to 17.0 g./l. Stated another way, at a concentration of 400 g./l. of CrOs, the said sum may vary from 2.7 to 9.0 g./l., and at a concentration of 800 g./l. of CrOs, the sum may vary from 4.4 to 17.0 g./1. The following table illustrates the relation between CrOa and catalyst concentrations.

Patented July 23, 1957 1 ice The preferred CrOa concentration is 500 to 700 g./l., for which range the preferred sulfate concentration varies from 0.6 to 5.3 g./l., the silicofluoride concentration varies from 1.0 or 1.5 to 12.0 g./l., the sum of the catalysts is 3.1 to 4.0 g./l. at the lower limit and 9.0 to 12.5,g./l. at the upper limit, and the temperature is to F.

The bath solution may be made up in several ways. In one way, which is preferred, the solution is made up from chromic acid, two catalyst-supplying compounds, namely, strontium sulfate and an alkali metal silicofluoride, and two soluble non-catalytic compounds one of which is a strontium compound and the other of which is an alkali metal compound. The alkali metal of the catalytic and non-catalytic compounds is the same and is selected from the class consisting of sodium and potassium. The non-catalytic compounds have the effect of controlling the concentrations of the dissolved sulfate and silicofluoride radicals in the bath by influencing the solubility of the salts used to introduce the radicals into the bath. Specific non-catalytic compounds are strontium carbonate, strontium oxide, strontium chromate, stron tium hydroxide, potassium hydroxide, potassium bichromate, potassium carbonate, potassium chromate, sodium bichromate, sodium carbonate, sodium hydroxide, sodium chromate. The amount of the strontium sulfate and of the alkali metal silicofluoride, is, in each case, sufficient to saturate the bath with dissolved sulfate and silicofluoride and to provide an insoluble residue of each salt in the bath. The soluble non-catalytic strontium and alkali metal compounds are each present in an amount sufficient to adjust or suppress the concentrations of the strontium sulfate and alkali metal silicofluoride, respectively, in solution in the bath from the unsuppressed saturation concentrations of the latter two compounds to lower concentrations. corresponding to the sums of sulfate and silicofluoride noted in Table 1 above.

An advantage of the foregoing bath solution is that it is self-regulating with respect to the concentrations of dissolved sulfate and dissolved silicofluoride.

Another way for making a suitable bath solution is like that just described except that the non-catalytic strontium compound is omitted. The amount of the strontium sulfate and the alkali metal silicofluoride, is, in each case, sufficient to saturate the bath with dissolved sulfate and silicofluoride and to provide an insoluble residue of each salt in the bath. The soluble non-catalytic alkali metal compound is present in an amount suflicient to adjust or suppress the concentration of the alkali metal A further way for making a suitable bath solution is one in which only the dissolved sulfate concentration is adjusted by means of a non-catalytic compound. Strontium sulfate and an alkali metal silicofluoride are used alone, as in the case of sulfuric and hydrofiuosilicic acids, or in combination with one or more other readily or less soluble compounds; and compounds of intermediate solubility may also be used alone or in combination with as the catalyst-supplying compounds, each in an amount other compounds. In making up or replenishing a nonsuflicient to saturate the bath with dissolved sulfate and suppressor type bath, the use of a catalyst-supplying comdissolved silicofluoride and to provide an insoluble resipound which would act as a suppressor in regard to due of each salt in the bath. A soluble non-catalytic another catalyst-supplying compound is omitted; for exstrontium compound is added in an amount sutficient to ample, if potassium silicofluoride is present in the bath or adjust or suppress the concentration of the strontium sulis used to supply silicofluoride, then potassium sulfate fate in solution in the bath from the unsuppressed saturas no used as the source of supply of sulfate, a salt like tion concentration of the latter to a'lower concentration sodium sulfate being used instead.

such that the sum of the dissolved sulfate and dissolved The unplated steel y have a hardness f at least 2 6 silicofluoride corresponds to the ranges set forth in Table ell, the upper limit being as des re y 1. The concentration of dissolved sulfate thus becomes Good results y e Secured if the hardness is in the the suppressed saturation concentration and that of disrange 0t 28C to 47C, Particularly 33C to 47C Rockwellsolved silicofluoride is the unsuppressed saturation con- Alley high q y Steels are Sultable Plating y centration. As before, in this bath the concentrations of the Present method, but the invention ls not limited to dissolved lf t and dissolved ili fl id are 1f them. Substantial thicknesses of chromium may be deregulated, but only the sulfate concentration is controlled posited 011 the Steel, being at least 2 mile and usually by a nomcatalytic compound ranging from 2 to 15 mils, but often going up to 30 mils,

It is apparent f the two Preceding types f b h or to whatever greater thickness that may be desired. solution that either the sulfate or the silicofiuoride con- Plating times y y Widely, for example from 1 to 24 centration may be regulated by means of a suitable nonhours more- Good cathode current efllclencles are catalytic compound of the kind above set forth. 25 obtainable.

Still another way for making a suitable bath solution Following Plating, the Steel article is baked at a is one like either of the two just described but in which Peratufe Of 200 to for about 1 t0 4 hours, ustlally no Suppressor or nomcatalytic compounds are d for 2 hours. For steels having lower draw temperatures sides chromic acid, the solution comprises two soluble the Preferred baking temperature is 350 to The catalyst-supplying compounds, namely, a sulfate radical t'atlgue Strength of the resulting chromium P baked bearing compound and a silicofiuoride radical bearing Steel article may be at least 85% 0f the Strength of the compound Compounds f varying solubility may be original unplated steel. Strengths of at least 70% of used, including those readily soluble in the bath solution, the l-mPlated steel are readily obtainable While strengths those sparingly soluble therein, and those of intermediate of at least 60% are Satisfactory for many Purposes The solubility, all of which compounds are intended herei chromium plate after baking has a whitish, satiny apas being soluble compounds. Specific sulfate-supplying peal'ancecompounds include readily soluble compounds like Several illustrative baths are described in the following furic acid, sodium sulfate, potassium sulfate, chromium examples, including Plating condltlonssulfate; sparingly soluble compounds like strontium sulfate; and compounds of intermediate solubility such as 40 Examples 1 to 8 calcium sulfate. Specific silicofluoride-supplying com- A number of plating baths were made up, the C103 pounds include read1ly soluble compounds like hydroand catalyst content of which are set forth below in Table fiLlOSlllClC acid and magnesium silicofiuoride; sparingly 2. The baths of Examples 1-7 are mixed catalyst baths soluble compounds like potassium silicofluoride; and comwhile that of Example 8 is a standard or conventional pounds of intermediate solubility like sodium silicofluosulfate bath included for comparison. For the bath of ride. After the bath solution has been initially made up Example 2 there was dissolved in water a dry mixture to the required concentrations, as indicated in Table 1, comprising chromic acid, sodium bichromate, sodium the catalyst content is maintained by dissolving catalystsilicofluoride, and strontium sulfate, so that the resulting supplying compounds in the bath as required, that is, in r solution had the composition set forth in Table 2. The amounts equivalent to the amounts of catalyst radicals to standard sulfate bath of Example 8 was prepared by disbe replaced. Compounds that are readily soluble may solving chromic and sulfuric acids in water. The baths require more frequent addition than those sparingly of the other examples were prepared by dissolving in soluble since the latter may sometimes be added in excess water a mixture comprising chromic acid, sodium silicoso as to saturate the bath and to provide an undissolved fluoride or magnesium silicofluoride hexahydrate and residue. The bath analysis should be available when sodium sulfate; in some instances hydrofiuosilicic acid required in order to determine when additions are neceswas used in place of magnesium silicofiuoride hexahydrate sary. When solutions of readily soluble catalyst-supply- .and sulfuric acid in place of sodium sulfate. Steel test mg compounds, such as sulfuric acid and hydrofluosilicic specimens were plated in the various baths under the conacid, are used to replenish the bath, they should, of course, ditions noted in the table. These fatigue test specimens be of know strength. So long as the concentration rewere the same, prepared from one-half inch round bar quirements of Table 1 are satisfied, sparingly soluble stock of SAE 4140 steel. Each specimen was 3%," long compounds may be employed alone or in conjunction and had a central portion that was reduced to a diameter with one or more other more soluble compounds, it being of A". The specimens had been heat-treated had a evident that the former will not always be used in saturahardness of Rockwell C43, a fatigue strength of 109 000 tion amounts; readily soluble compounds are useful either p. s. i., and a tensile strength of 222,500 p. s. i. 7

TABLE 2 Exam 1e De oslt OrO SO l its $83 $2 it t3 lt 2 13' 2g 2 g. g 2. 6F 6. 9 3 l7. 5 71 600 5:3 130 a 11 8.8 3 13 3 3 $88 g; 11 1 11. 7 130 3 11 o 250 215 0' i1: 2

Fatigue strength data of several of the chromium plated specimens are given in Table 3 together with the plate thickness. The fatigue tests were performed with a rotating beam fatiguemachine of the R. R. Moore type.

Unless otherwise specified, the reference in the claims to the CrOa content of the baths is intended to include the amount of CrOs added per se and the amount added in the form of a chromate or dichromate salt.

In the light of the foregoing description, the following is claimed:

1. A method for reducing the loss of fatigue strength following chromium plating of an article of hard steel comprising electrodepositing chromium on the article by passing a current of 1 to 6 a. s. i. to said article as cathode in an aqueous chromium plating bath at a temperature of 120 to 170 F., said bath comprising essentially 400 to 800 g./l. of CrOs, a sulfate-containing compound and a silicofluoride-containing compound each in an amount to provide, as catalysts, 0.5 to 6.0 g./l. of dissolved sulfate, SO4=, and 0.1 to 16.0 g./l. of dissolved silicofluoride, SiFs=, respectively, the sum of dissolved sulfate and dissolved silicofluoride varying with the Cl'Os concentration as follows: as the CrOa increases from 400 to 800 g./l., the lower limit of said sum increases linearly from 2.7 to 4.4 g./l. and the upper limit of said sum increases linearly from 9.0 to 17.0 g/L, plating chromium on the article to form a coating at least 2 mils thick, heating the plated article to a temperature of 200 to 800 F. for a sufficient duration to produce a chromium plated article of said hard steel having a fatigue strength of at least 60% of that of the original unplated steel.

2. A method for reducing the loss of fatigue strength following chromium plating of an article of steel having a hardness of at least C Rockwell and a fatigue strength of at least 50,000 p. s. i., comprising electrodepositing chromium on the article by passing a current of 1 to 6 a. s. i. to said article as cathode in an aqueous chromium plating bath at a temperature of 120 to 170 F., said bath comprising essentially 400 to 800 g./l. of ClOs, a sulfate-containing compound and a silicofluoride-containing compound each in an amount to provide, as catalysts, 0.5 to 6.0 g./l. of dissolved sulfate, SO4=, and 0.1 to 16.0 g./l. of dissolved silicofluoride, SiFe=, respectively, the sum of dissolved sulfate and dissolved silicofluoride varying with the CrOs concentration as follows: as the CrOs increases from 400 to 800 g./l., the lower limit of said sum increases linearly from 2.7 to 4.4 g./l. and the upper limit of said sum increases linearly from 9.0 to 17.0 g./l., plating chromium on the article to form a coating at least 2 mils thick, heating the plated article to a temperature of 200 to 800 F. for about 1 to 4 hours, and thereby producing a chromium plated article of said hard steel having a fatigue strength of at least 60% of that of the original unplated steel.

3. A method for reducing the loss of fatigue strength following chromium plating of an article of steel having a hardness of 33C to 47C Rockwell and a fatigue strength of 70,000 to 110,000 p. s. i., comprising electrodepositing chromium on the article by passing a current of 1 to 6 a. s. i. to said article as cathode in an aqueous chromium plating bath at a temperature of to F.-, said bath comprising essentially 500 to 700 g./l. of CrO3, a sulfate containing compound and a silicofluoride-containing compound'eachin an' amount to provide, as catalysts, 0.6 to 5.3 g./l. of dis'solvedsulfate, SO4=, and 1.0 to 12.0 g./l. of dissolved silicofluoride, SiFs=, respectively, the sum of dissolved sulfate and dissolved silicofluoride varying with the C103 concentration as follows: as the CrOs increases from 500 to 700. g./l., the lower limit of said sum increases linearly from 3.1 to 4.0 g./l. and the upper limit of said sum increases linearly from 9.0 to 12.5 g./l., plating chromium on the article to form a coating about 2 to 15 mils thick, heating the plated article to a temperature of 350 to 750 F. for about 2 hours, and thereby producing a chromium plated article of said hard steel having a fatigue strength of at least 85% of that of the original unplated steel.

4. A method according to claim 1 in which said bath is free of compounds acting to suppress the concentra tions of dissolved sulfate and dissolved silicofluoride.

5. A method according to claim 1 in which said sulfateand silicofluoride-containing compounds are strontium sulfate and an alkali metal silicofluoride, respectively, each present in an amount suflicient to provide an undissolved excess thereof in said bath, and in which said bath contains a soluble non-catalytic strontium compound and a soluble non-catalytic alkali metal compound acting to suppress the concentrations of dissolved sulfate and dissolved silicofluoride so that the sum of dissolved sulfate and dissolved silicofluoride is within said upper and lower limits recited in claim 1, and the alkali metal of said alkali metal compound being the same as that of said alkali metal silicofluoride.

6. A method according to claim 1 in which said sulfateand silicofluoride-containing compounds are strontium sulfate and an alkali metal silicofluoride, respectively, each present in an amount sufiicient to provide an undissolved excess thereof in said bath, and in which said bath contains a soluble non-catalytic alkali metal compound acting to suppress the concentration of dissolved silicofluoride so that the sum of dissolved sulfate and dissolved silicofluoride is within said upper and lower limits recited in claim 1, and the alkali metal of said alkali metal compound being the same as that of said alakil metal silicofluoride.

7. A method according to claim 1 in which said sulfateand silicofluoride-containing compounds are strontium sulfate and an alkali metal silicofluoride, respectively, each present in an amount sufiicient to provide an undissolved excess thereof in said bath, and in which said bath contains a soluble non-catalytic strontium compound acting to suppress the concentration of dissolved sulfate so that the sum of dissolved sulfate and dissolved silicofluoride is within said upper and lower limits recited in claim 1.

8. A method according to claim 1 in which said sulfateand silicofluoride-containing compounds are strontium sulfate and an alkali metal silicofluoride, respectively, each present in an amount sutficient to provide an undissolved excess thereof -in said bath, and in which said bath contains a soluble non-catalytic compound acting to suppress the concentration of one of said dissolved catalyst-providing compounds so that the sum of dissolved sulfate and dissolved silicofluoride is within said upper and lower limits recited in claim 1, said noncatalytic compound being selected from the class consist-ing of a strontium compound and an alkali metal compound, and the alkali metal of said alkali metal compound being the same as that of said alkali metal silicofluoride.

9. The method of claim 1 wherein the steel of the unplated article has a hardness of at least 28C Rockwell.

10. The method of claim 1 wherein the steel of the unplated article has a hardness of 33C to 47C Rockwell,

11. The method'of claim 1 wherein the fatigue strength of the steel of said unplated article is at least 60,000,

p. s. 1. i

12. The method of claim 1 wherein chromium is plated on the article to form a coating 5 to 15 mils thick.

13. The method of claim 1 whereinsaid plated article is heated at a temperature of 350 to 800? F. for about 2 hours.

14. The method of claim 1 wherein the chromium plated article of steel that is produced has a fatigue 10 strength of at least 70% of that of the unplated original steel.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF REDUCING THE LOSS OF FATIGUE STRENGTH FOLLOWING CHROMIUM PLATING OF AN ARTICLE OF HARD STEEL COMPRISING ELECTRODEPOSITING CHROMIUM ON THE ARTICLE BY IN AN AQUEOUS CHROMIUM PLATING BATH AT A TEMPERATURE OF 120 TO 170* F., SAID BATH COMPRISING ESSENTIALLY 400 TO 800 G./1. OF CRO3, A SULFATE-CONTAINING COMPOUND AND A SILICOFLUORIDE-CONTAINING COMPOUND EACH IN AN AMOUNT TO PROVIDE, AS CATALYSTS, 0.5 TO 6.0 G./1. OF DISSOLVED SULFATE, SO4=, AND 0.1 TO 16.0 G./1. OF DISSOLVED SILICOFLUORIDE, SIF6=, RESPECTIVELY, THE SUM OF DISSOLVED SULFATE AND DISSOLVED SILICOFLUORIDE VARYING WITH THE CRO3 CONCENTRATION AS FOLLOWS: AS THE CRO3 INCREASES FROM 400 TO 800 G./1., THE LOWER LIMIT OF THE SAID SUM INCREASES LINERALY FROM 2.7 TO 4.4 G./1. AND THE UPPER LIMIT OF THE SAID SUM INCREASES LINEARLY FROM 9.0 TO 17.0 G/1.,PLATING CHROMIUM ON THE ARTICLE TO FORM A COATING AT LEAST 2 MILS THICK, HEATING THE PLATED ARTICLE TO A TEMPERATURE OF 200 TO 800* F. FOR A SUFFICIENT DURATION TO PRODUCE A CHROMIUM PLATED ARTICLE OF SAID HARD STEEL HAVING A FATIGUE STRENGTH OF AT LEAST 60% OF THAT OF THE ORIGINAL UNPLATED STEEL. 